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CPD: Design and specification of lead sheet roofing

This is the last of three CPDs sponsored by the Lead Sheet Association (LSA). It covers rolled lead sheet finishes to flat and pitched roofs, including fixings, gutters and ventilation details.

Previous CPDs covered the general use of lead sheet in buildings (AJS 08.12) and its use in vertical cladding (AJS 09.12).

The LSA provides technical advice on the use and application of rolled lead sheet to the construction industry and a wide range of leadwork installation courses, including QCF (Qualification & Credit Framework) Levels 2 & 3.

The LSA’s 203-page Rolled Lead Sheet, The Complete Manual is available through the association’s website, www.leadsheet.co.uk.

Part 1: Flat roof bays

When covering roofs in lead sheet, it is vital to limit thermal and other movement that may cause fatigue cracking. The larger the piece of lead used, the greater the risk of movement and cracking. For this reason, lead sheet roofing is divided into bays or panels. The size of the bays, and hence the spacing between the joints, depends on the code or thickness of lead used. Minimum fall for a flat roof is 1 in 80. For flat and low-pitched roofs up to 10°, maximum bay sizes are given on page 102 of the Manual. Bays may occasionally be increased in width, but never in length or area. Extra thickness may be required in positions that might be prone to wind lift or foot traffic, or which involve complicated intersections. Joints in the direction of the fall should generally be wood-cored rolls. Hollow rolls should only be used where there is no risk of damage by foot traffic. Joints across the fall should always be stepped drips. Rolls can be laid in line or staggered at drips.

Part 2: Fixing flat roofs

Lead sheet bays are fixed with copper or stainless steel clout nails. But bays should never be nailed all round the perimeter, as this would restrict thermal movement. For roofs up to 3° in pitch, each bay should be nailed along its top edge into a shallow rebate in the drip at 50mm centres. At both sides it is nailed to the wood rolls at 150mm centres for just the top third of its length. At abutments, the top edge is turned up against the wall without nailing, while the spacing of nails down the top third of the sides is increased to 50mm centres. All nail heads are later overlapped by adjacent sheets. To secure roofs between 3° and 10° against creep, the top edge requires a double row of nails at 75mm intervals, but it is not rebated into the drip. Nail heads need to be sealed against water infiltration with solder dabs or lead welding. No nails are needed down the sides of the bay.

Part 3: Flat roof details

Joints and other details in flat roofs are intricate and call for expert hand craftsmanship, particularly at complex intersections at ridges, hips and valleys. Stronger, more durable details are prepared on the bench and welded on site, rather than being bossed with wooden mallets. At an abutment on pitches up to 3°, each bay is turned up by 100mm against the wall and covered by at least 75mm with a flashing. On roof pitches between 3° to 10°, the flashing is extended to cover nail heads. In both cases, flashings are secured with lead clips.

Part 4: Pitched roofs

For roofs with pitches of 10° to 80°, laps can be used for joints across the fall, whereas for flat and low-pitched roofs, stepped drips must be used. For joints running along the fall in pitched roofs, wood rolls or hollow rolls are more weatherproof and robust than welts or standing seams. Maximum bay sizes for pitched roofs are given in Tables 12 and 13 on page 136 of the Manual. Laps should be at least 75mm measured vertically from the bottom edge to the first row of nails, though the actual length can stretch up to 395mm on a shallow 11° pitch. Laps are usually staggered from bay to bay to avoid build-up of thickness at two-way intersections.

Part 5: Fixing pitched roofs

For pitched roofs, lead sheet bays are usually fixed with two rows of nails at the head and loosely clipped at the bottom. Where Code 7 & 8 lead sheet is used on roofs over 60°, three rows of nails are recommended. Where wood rolls are used, the top third is also nailed. Where a roof of 11° to 30° in pitchmeets an abutment, the bay’s top edge is turned up 50mm and the nail heads are sealed before being covered with a flashing. For pitches over 30°, no upstand or sealing of nails is necessary, provided the flashing laps the top of the bay by at least 75mm in vertical height.

Part 6: Valley gutters

Where two areas of roofing meet at a valley, a shallow 150mm box gutter is required. For flat and low-pitched roofs, the gutter should be 50mm deep if its lining is turned into a rebate in the roof slope on either side, or 60mm deep if nailed along its vertical sides. It also requires stepped drips, ideally lined up with the drips across the roof on either side. On pitches of 10° to 20°, the sides of the valley gutters are not fixed and laps are used instead of drips. On pitches over 20°, only a simple V-shaped valley gutter is needed, rather than an elaborately recessed gutter.

Part 7: Box & tapered gutters

Extended horizontal lead-lined gutters can be tapered with splayed sections or box gutters. As with flat roofs, the lead sheet bays should be separated by stepped drips. Height of drips and spacing is the same as for flat roofs (see page 89 of the Manual). However, using a thicker than minimum code can sometimes be more economical if fewer drips are required. A lead gutter should be 100-225mm deep, with its upstands lapped by lead flashings, lead roof covering or roofing underlay. The girth, which is measured as if the gutter were laid out flat, should not exceed 750mm for Code 4 and 1,000mm for Code 8. Wider gutters can be divided along their length by wood rolls.

Part 8: Ventilation details

Care should be taken to avoid condensation or water ingress below lead sheet roofing, as this could cause deterioration of the substrate and corrosion of the lead sheet itself. To dissipate any moisture, a warm ventilated roof is recommended with thermal insulation laid above a vapour control layer and below a breather membrane and ventilated air space. To ventilate the air space, many variants of details can be fashioned to incorporate the lead sheet covering, without resorting to proprietary vents.

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